1. Field of Invention
The present invention relates to a method for etching poly Si gate stacks with raised shallow trench isolation (STI) on a substrate where the thickness of the poly Si gates at the AA and STI oxide is different.
2. Description of the Related Art
In contemporary Complementary Metal Oxide Semiconductor (CMOS) technology, field effect transistors are employed that are adjacent to or bounded by trenches to affect semiconductor isolation techniques. More particularly, the isolation is referred to as shallow trench isolation (STI), which is an isolation technique in which the insulating material or trench extends beyond or is raised above the surface of the substrate on top of the gate stack. Even more particularly, it relates to an isolation structure for a transistor in a DRAM (dynamic random access memory) cell.
Designers of technologies for producing semiconductor devices have been continually inspired to increase device densities to remain cost and performance competitive. As a result, VLSI and ULSI technologies entered the sub-micron realm of structural dimensions and are designing technologies in the nanometer feature size range. In the future atomic physical limits will be reached in the conventional two-dimensional design approach to semiconductor device design.
Dynamic Random Access Memory (DRAM) designers have met the challenges in advancing technologies by pushing the limits of feature size resolution with each generation of DRAM. Thus, designers of 64 K bit DRAMs ere perplexed to learn that a practical physical limit to charge capacity of storage capacitors had already been reached due to the minimum charge capacity required to allow reliable data signal sensing in the presence of naturally occurring atomic particle radiation inherently present in fabrication materials and the operating environment.
Storage capacitors in the range of about 50 femtofarads are now considered to be a physical limit. This limitation prevented a continuation of the scaling of DRAM dimensions and voltages initiated in the early 1970s. Reduction in the surface area of semiconductor substrate utilized by the DRAM storage capacitor has been severely restricted. Due to decreases in the thickness of reliable capacitor dielectric materials, existing 1 Megabit (1 Mb) DRAM technologies continue to enjoy the freedom of planar, two-dimensional device and circuit design.
Beginning with 4 Mb DRAMs, three-dimensional design has been utilized to the extent that the single device/capacitor memory cell has been altered to provide the capacitor in a vertical dimension.
In such designs, the capacitor is formed in a trench formed in the surface of the semiconductor substrate. In still denser designs, other forms of three-dimensional capacitors are proposed, such as stacking the plates of the capacitors above the transfer device. Such designs, however, present difficulties in forming the interconnections to the required word access and data bit lines to the DRAM memory cell. Additional designs have been proposed in which the transfer device and its associated capacitor are both formed within a trench of preferably minimum feature size.
A large number of proposals for 16 Mb and greater density DRAM cell designs have avoided continuing development of trench cell technology because of the existence of charge leakage mechanisms known to be present in trench capacitor structures. As these leakage mechanisms have become known, extensions of trench DRAM cells designs have been used successfully in 16 Mb designs.
“Trench and Compact Structures for DRAMs” by P. Chatterjee et al., International Electron Devices Meeting 1986, Technical Digest paper 6.1, pp. 128–131, disclose variations in trench cell designs through 16 Mb DRAM designs, including the Substrate Plate Trench (SPT) cell disclosed in U.S. Pat. No. 4,688,063. The SPT cell uses a highly conductive substrate as the DRAM cell plate. The storage node of each cell is formed in a deep trench in the substrate.
U.S. Pat. No. 4,801,988 disclose an improved SPT cell which includes a thick isolation region formed within the trench to enable higher density packaging of DRAM cells. The article “CMOS Semiconductor Memory Structural Modification to Allow increased Memory Charge” anonymous, IBM Technical Disclosure Bulletin, Vol. 31, No. 11, April 1989, pp. 162–5, disclose a method of isolating the substrate plate of an SPT cell from support devices by providing a buried region under support devices in order to allow the plate reference voltage to be separately biased at an optimum Vdd/2 volts.
U.S. Pat. No. 4,829,017 disclose a method of forming a buried doped layer in a substrate by forming a shallow trench, protecting its sidewalls, further extending the trench and doping the walls of the extended trench to form a continuous doped region useful as the storage node of a trench DRAM.
U.S. Pat. No. 4,918,502 relates to variations in SPT DRAM cells in which a buried region of opposite conductivity type from the substrate is used on one plate of the DRAM storage capacitor. A buried plate trench DRAM cell in which the storage node of the cell and a sheath plate are formed in a single trench is disclosed. At the bottom of the trench a diffusion of opposite type from the substrate is formed such that the diffusions of adjacent cells interconnect forming a grid-like structure. One or more trenches not associated with a DRAM cell is formed to act as a reach through to enable the doped regions to be biased at a suitable reference voltage.
A severe disadvantage for raised STI structures is that the thickness of the poly Si gates at the AA and the STI oxide is different. This difference in thickness of poly Si gates at the AA and the STI oxide makes it very difficult during the manufacturing process to obtain vertical profiles of the poly Si gate stacks both at the AA and at the STI oxide.
While etching the poly Si gate with raised STI structure, poly Si notching is typically observed at the STI oxide and poly Si tapering is typically observed at the AA.
Therefore, there is a need in the art of semiconductor isolation techniques, more particularly, in shallow trench isolation (STI) in which insulating material is raised above the surface of the semiconductor—for a method to eliminate the different thickness and thereby eliminate or substantially reduce poly Si notching at the STI oxide and poly Si tapering at the AA or both.